Physicians, patients and policy-makers are influenced not only by the results of studies but also by how authors present the results. Depending on which measures of effect they choose, the impact of an intervention may appear very large or quite small, even though the underlying data are the same. Some clinicians use the number needed to treat (NNT) when considering the consequences of treating or not treating. This tip explains the concept and calculation of number needed to treat in a variety of ways.